Conventional, oil recovery involves drilling a well and pumping a mixture of oil and water from the well. Oil is separated from the water and the water is usually injected into a sub-surface formation. Conventional recovery works well for low viscosity oil. However, conventional oil recovery processes do not work well for higher viscosity, or heavy, oil.
Enhanced oil recovery processes employ thermal methods to improve the recovery of heavy oils from sub-surface reservoirs. The injection of steam into heavy oil bearing formations is a widely practiced enhanced oil recovery method. Typically, several tonnes of steam are required for each tonne of oil recovered. Steam heats the oil in the reservoir, which reduces the viscosity of the oil and allows the oil to flow to a collection well. After the steam fully condenses and mixes with the oil the condensed steam is classified as produced water. The mixture of oil and produced water that flows to the collection well is pumped to the surface. Oil is separated from the water by conventional processes employed in conventional oil recovery operations.
For economic and environmental reasons it is desirable to recycle the water used in the steam injection. This is accomplished by treating the produced water and directing the treated feedwater to a steam generator or boiler.
Several treatment processes are used for converting produced water into steam generator or boiler feedwater. These processes typically remove constituents which form harmful deposits in the boiler or steam generator. These water treatment processes used in steam injection enhanced oil recovery typically do not remove all dissolved solids, such as sodium and chloride.
Water treatment is a necessary operation in heavy oil recovery operations. This is because in order to recover heavy oil from certain geologic formations, steam is required to increase the mobility of the oil in the formation. Traditionally, heavy oil recovery operations have utilized “once through” type steam generators. The steam is injected via injection wells to fluidize the heavy oil. Different percentages of water and steam can be injected into the injection wells. The decision to vary the percentages of water and steam to be injected into the injection well depend a variety of factors including the expected output of oil and the economics of injecting different water/steam mixtures. An oil/water mixture results, and the mixture is pumped to the surface. Then, the sought-after oil is separated from the water and recovered for sale.
The produced water stream, after separation from the oil, is further de-oiled, and is treated for reuse. Most commonly, the water is sent to the “once-through” steam generators for creation of more steam for oil recovery operations. The produced water stream is typically required to have less than about 8000 PPM TDS (as well as meeting other specific constituent requirements) for re-use. Thus, in most cases, the recovered water must be treated before it is sent to the steam generators. Normally, such treatment is initially accomplished by using a warm lime softener, which removes hardness, and which removes some silica. Then, an “after-filter” is often utilized, to prevent carry-over of any precipitate or other suspended solids. For polishing, in a hardness removal step, a weak acid cation (WAC) system is often utilized to simultaneously remove hardness and the alkalinity associated with the hardness.
A relatively new heavy oil recovery process, referred to as the Steam Assisted Gravity Drainage heavy oil recovery process (the “SAGD” process), ideally utilizes 100% quality steam for injection into wells (i.e., no liquid water). Initially, water utilized for generating steam in such operations can be treated much the same as in the just discussed traditional heavy oil recovery operations. However, in order to produce 100% quality steam using a once-through type steam generator, a series of vapor-liquid separators are required to separate the liquid water from the steam. The 100% quality steam is then sent down the well and injected into the desired formation.
Another method for generating the required 100% quality steam involves the use of packaged boilers. Various methods are well known for producing water of sufficient water to be utilized in a packaged boiler. One method which has been developed for use in heavy oil recovery operations involves de-oiling of the produced water, followed by a series of physical-chemical treatment steps. Such additional treatment steps normally include such unit operations as warm lime softening, after-filtration, organic traps, pre-coat filters or ultrafiltration, reverse osmosis, and mixed bed demineralization. Such a physical-chemical treatment system may have a high initial capital cost, and generally involves significant ongoing chemical costs. Moreover, there are many waste streams to discharge, involving a high sludge disposal cost. Further, where membrane systems such as ultrafiltration or reverse osmosis are utilized, relatively frequent membrane replacement is encountered, at significant additional cost. Also, such processes can be quite labor intensive to operate and to maintain. Therefore, it is clear that the development of a simpler, more cost effective approach to produced water treatment as necessary for packaged boiler make-up water would be desirable.
In summary, the currently known and utilized methods for treating heavy oil field produced waters in order to generate high quality steam for down-hole are not entirely satisfactory because: most physical chemical treatment systems are quite extensive, are relatively difficult to maintain, and require significant operator attention; they often require liquid-vapor separation equipment, which adds to equipment costs; a large quantity of unusable hot water is created, and the energy from such water must be recovered, as well as the water itself, in order to maintain an economic heat and material balance in plant operations; they require large amounts of expensive chemicals, many of which require special attention for safe handling, and which present safety hazards if mishandled; the treatment train produces fairly substantial quantities of undesirable sludges and other waste streams; the disposal of waste sludges and other waste streams is increasingly difficult, due to stringent environmental and regulatory requirements.
Thus, it can be appreciated that it would be advantageous to provide a new process which minimizes the production of undesirable waste streams, while minimizing the overall costs of owning and operating a heavy oil recovery plant by eliminating the water treatment system and conventional boilers with a single system.